1. Field of the Invention
The present invention relates generally to an ignition timing control apparatus for a spark ignition type engine, which controls the ignition timing of an air-fuel mixture in the combustion chamber in accordance with the running condition of the engine. More particularly, this invention relates to an ignition timing control apparatus for an engine, that corrects the ignition timing in accordance with the presence or absence of exhaust gas that is discharged from the combustion chamber and is recirculated to the combustion chamber by an exhaust gas recirculation (EGR) apparatus.
2. Description of the Related Art
The ignition timing of the air-fuel mixture in spark ignition type engines is one of the factors that greatly affects emissions, fuel economy, driving characteristics and the like. It is therefore necessary to optimally control the ignition timing in accordance with the running condition of the engine. A known apparatus that performs such control is equipped with a computer. This computer computes the value of the optimal ignition timing in accordance with the engine running condition and activates the ignition plugs based on the computed value. When the engine is under normal (steady) operation, the computer in this type of control apparatus selects the value of the optimal ignition timing from previously stored values based on the engine speed and the load of the engine.
There is an exhaust gas recirculation (EGR) apparatus provided in the engine. This EGR apparatus recirculates part of the exhaust gas, which is discharged from the combustion chamber of the engine, into the air-fuel mixture that is to be drawn into the combustion chamber. When this exhaust gas enters the combustion chamber and the inactive gas in the exhaust gas acts on the burning air-fuel mixture, the combustion temperature of the air-fuel mixture falls. As a result, the amount of nitrogen oxide (NO.sub.x) in the exhaust gas decreases.
When the amount of the exhaust gas to be recirculated by the EGR apparatus or the EGR amount increases, the propagation of burning of the air-fuel mixture in the combustion chamber is apt to slow. It is therefore necessary to advance the value of the ignition timing in accordance with the increase in EGR amount in an engine equipped with the EGR apparatus. Japanese Unexamined Patent Publication No. Sho 59-221467 discloses an apparatus which controls the ignition timing in association with a change in EGR amount. (This apparatus will be hereinafter referred to as "first prior art".) At the time the EGR apparatus recirculates the exhaust gas into the combustion chamber, this type of control apparatus adds the advance angle value, determined in accordance with the EGR amount, to a reference value (base value) of the ignition timing, which is used when the exhaust gas is not recirculated. Consequently, the ignition timing value to be finally transmitted is computed.
There is another type of ignition timing control apparatus which controls the ignition timing to the vicinity of the timing at which knocking starts in order to obtain the maximum output torque from the engine. This type of control apparatus has a knock sensor which detects the vibration of the engine. Based on the value detected by the knock sensor, the computer determines if knocking has occurred. The computer retards the ignition timing by a desired amount based on the result of the determination. If the maximum retard angle is set to be always constant regardless of the running condition of the engine, however, this control apparatus may set the retard angle value to a very large value when the engine runs in a high-load condition. In this case, the temperature of the exhaust gas is likely to increase rapidly.
Japanese Patent Publication No. Sho 62-24627 discloses an ignition timing control apparatus which overcomes the above problem (hereinafter referred to as "second prior art"). When determining that knocking has occurred in the engine, this control apparatus controls the ignition timing, which is determined in accordance with the running condition of the engine, in such a way as to retard the ignition timing to near the limit beyond which knocking starts. This control apparatus determines the maximum value of the retard angle in accordance with the engine speed and the load of the engine. Further, the control apparatus limits the change in retard angle to a value no greater than the determined maximum value.
When the control apparatus according to the second prior art is adapted for an EGR apparatus, the ignition timing should be corrected in accordance with the EGR amount as in the case of the control apparatus according to the first prior art. In this case, however, the control apparatus adds the advance angle value according to the EGR amount to the base value of the ignition timing at the time the exhaust gas is recirculated by the EGR apparatus. Further, the control apparatus determines the maximum retard angle with respect to the result of the addition. Therefore, knocking is more likely to occur due to the rise of the temperature of the air-fuel mixture due to the recirculated exhaust gas. Therefore the maximum value of the retard angle should be large. Nevertheless, the maximum value of the retard angle is reduced. Consequently, the value of the ignition timing, which is limited based on the maximum value of the retard angle, may be too far advanced, and thus, knocking is likely to occur.